Radiation-sensitive elements and their storage stability

ABSTRACT

Radiation-sensitive element comprising (a) a substrate with at least one hydrophilic surface and (b) a radiation-sensitive coating on at least one hydrophilic surface of the substrate, wherein the coating comprises: (i) at least one free-radical polymerizable monomer and/or oligomer and/or polymer with at least one ethylenically unsaturated group each, (ii) at least one absorber selected from photoinitiators and sensitizers, which is capable of absorbing radiation of a wavelength in the range of 250 to 1,200 nm and (iii) at least one stabilizer comprising in its molecule at least one group capable of inhibiting free-radical polymerization, and at least one other group capable of sorption at the hydrophilic surface of the substrate.

The present invention relates to radiation-sensitive elements withexcellent storage stability, in particular radiation-sensitive elementscomprising a stabilizer in the radiation-sensitive coating, which can besorbed at the surface of the substrate. The invention furthermorerelates to a process for the production of such elements and to alithographic printing form.

The technical field of lithographic printing is based on theimmiscibility of oil and water, wherein the oily material or theprinting ink is preferably accepted by the image area, and the water orfountain solution is preferably accepted by the non-image area. When anappropriately produced surface is moistened with water and a printingink is applied, the background or non-image area accepts the water andrepels the printing ink, while the image area accepts the printing inkand repels the water. The printing ink in the image area is thentransferred to the surface of a material such as paper, fabric and thelike, on which the image is to be formed. Generally, however, theprinting ink is first transferred to an intermediate material, referredto as blanket, which then in turn transfers the printing onto thesurface of the material on which the image is to be formed; thistechnique is referred to as offset lithography.

A frequently used type of lithographic printing plate precursorcomprises a photosensitive coating applied onto a substrate on aluminumbasis. The coating can react to radiation such that the exposed portionbecomes so soluble that it is removed during the developing process.Such a plate is referred to as positive working. On the other hand, aplate is referred to as negative working if the exposed portion of thecoating is hardened by the radiation. In both cases, the remaining imagearea accepts printing ink, or is oleophilic, and the non-image area(background) accepts water, or is hydrophilic. The differentiationbetween image and non-image areas takes place during exposure, for whicha film is attached to the printing plate precursor under vacuum in orderto guarantee good contact. The plate is then exposed by means of aradiation source, part of which is comprised of UV radiation. When apositive plate is used, the area on the film corresponding to the imageon the plate is so opaque that the light does not affect the plate,while the area on the film corresponding to the non-image area is clearand allows light to permeate the coating, whose solubility increases. Inthe case of a negative plate, the opposite takes place: The area on thefilm corresponding to the image on the plate is clear, while thenon-image area is opaque. The coating beneath the clear film area ishardened due to the incident light, while the area not affected by thelight is removed during developing. The light-hardened surface of anegative working plate is therefore oleophilic and accepts printing ink,while the non-image area that used to be coated with the coating removedby the developer is desensitized and therefore hydrophilic.

Photosensitive mixtures have been used for years in photopolymerizablecompositions for the production of photosensitive materials such as e.g.printing plates. However, an improved sensitivity in particular in thevisible spectral range is required for new and advanced applications(e.g. exposure by means of lasers) so that the exposure time can beshortened. From an economic point of view it is also important thatlow-intensity lasers can be used, which are less expensive and morereliable than high-intensity lasers. Therefore, efforts have been madefor some time to increase the sensitivity of photosensitive mixtures tobe used in photopolymerizable compositions.

It is known that the free-radical polymerization of ethylenicallyunsaturated compounds can be initiated by irradiation with visible lightin the presence of photoreducible dyes and reducing agents, e.g. amines(U.S. Pat. No. 3,097,096). EP-A-122 223 discloses photoinitiators andphotopolymerizable compositions comprising metallocenes. The use of suchmetallocenes resulted in an increase in the sensitivity of thephotopolymerizable layer and thus a reduction of the necessaryirradiation time and the required power of the radiation source.Attempts were also made to obtain improved results from the use ofmetallocenes that had been modified further, e.g. in the documentsEP-A-401 165, U.S. Pat. No. 4,590,287, EP-A-255 486, EP-A-256 981 andU.S. Pat. No. 5,106,722.

Document DE-A-40 08 815 describes a photopolymerizable mixturecomprising a polymeric binder, a free-radical polymerizable compoundwith at least one polymerizable group and at least one photooxidizablegroup in the molecule and a metallocene compound as photoinitiator.

In order to further improve sensitivity, it was tried to use themetallocene compound together with a coinitiator. For example, EP-B-269573 discloses liquid mixtures of photoinitiators which are solutions oftitanocene compounds in liquid photoinitiators of the type α-hydroxy-and α-aminoacetophenone derivatives. DE-A-38 32 032 describes aphotopolymerizable mixture comprising a polymeric binder, a free-radicalpolymerizable compound with at least one polymerizable group, aphotoreducible dye, and, as initiator, a metallocene as well as acoinitiator. The coinitiator is a trihalogenomethyl compound divisibleby radiation, which is intended to increase photosensitivity. Compoundswith a triazine ring in the parent substance carrying twotrihalogenomethyl groups are preferred.

DE-A40 13 358 describes a specific process for the production ofprinting forms or photoresists using metallocene compounds asphotoinitiator, which is aimed at an improvement of sensitivity.

U.S. Pat. No. 3,717,558 describes metallocenes of subgroup elements incombination with a further photoinitiator comprising an activatedhalogen-containing group for the use in photopolymerizable recordingmaterials. However, these initiator combinations are very sensitive tooxygen and hydrolysis and are therefore not very suitable for theproduction of printing plates and resist materials.

The use of a combination of specific organometal compounds and oniumsalts in a hardening agent for polymerizable compounds is also known(U.S. Pat. No. 5,086,086). As metallocene compound, organometalcompounds are used whose essential feature is that they comprise atleast one metal-metal sigma bond, i.e. that at least twotransition-metal atoms are present in one complex. The hardening agentsdescribed in U.S. Pat. No. 5,086,086 are not used together with dyes forlight-induced polymerization.

U.S. Pat. No. 4,971,892 discloses photopolymerizable compositions whichare particularly suitable for printing plates and are said to exhibit ahigh degree of sensitivity to visible light. As initiator systems forfree-radical polymerization, these photopolymerizable compositionscomprise an initiator selected from diaryliodonium salts, halogenatedtriazines and triarylsufonium salts, as well as a specific merocyaninedye.

U.S. Pat. No. 4,959,297 is directed to photopolymerizable compositionscomprising at least one vinyl monomer capable of undergoing free-radicalpolymerization, a photoinitiator system, a diaryliodonium salt, apigment, one or more electron donating compounds and additives. Finally,DE-A-4,217,495 discloses a photopolymerizable mixture and a recordingmaterial produced therefrom.

DE-A-4,418,645 describes a photosensitive mixture comprising a binder,one or more polymerizable compounds with at least one polymerizablegroup and one or more dyes absorbing in the range of 250 nm to 700 nm,as well as an initiator system comprising at least one metallocenecompound and at least one onium compound.

WO 96/34314 describes negative working lithographic printing platescomprising an overcoat on their photosensitive coating, which overcoatcomprises a quenching agent bonded to a polymer which can deactivatefree radicals. The quenching agent preferably comprises a nitrylfunction such as e.g. a TEMPO group(2,2,6,6-tetramethyl-1-piperidinyl-oxy). However, the attained storagestability in combination with good sensitivity does not meet the highstandards required in practical applications.

The use of free-radical scavengers such as e.g. TEMPO inlight-hardenable compositions is furthermore known from the documentsU.S. Pat. No. 6,291,704, U.S. Pat. No. 6,308,001, US-A-01 0044481,US-A-01 0053502, US-A-02 0012516 and US-A-01 0034458. However, here aswell the balance between storage stability on the one hand andsensitivity and resolution on the other hand is not satisfactory.

JP-A-2001-222101 describes photosensitive compositions comprising anitroxyl compound and a sensitizer system comprising a borate complexand a dye. Various TEMPO derivatives are listed as suitable nitroxylcompounds.

It is the object of the present invention to provide novelradiation-sensitive elements exhibiting improved properties compared tothe ones known in the prior art, in particular high photosensitivity, ahigh degree of thermal stability, high resolution in combination withgood storage stability, and—in the case of printing plates—yielding alarge number of copies on the printing machine.

The problem underlying the invention is solved by a radiation-sensitiveelement comprising

-   -   (a) a substrate with at least one hydrophilic surface and    -   (b) a radiation-sensitive coating on at least one hydrophilic        surface of the substrate, wherein the coating comprises:        -   (i) at least one free-radical polymerizable monomer and/or            oligomer and/or polymer with at least one ethylenically            unsaturated group each,        -   (ii) at least one absorber selected from polymerization            initiators and sensitizers, which is capable of absorbing            radiation of a wavelength in the range of 250 to 1,200 nm,        -   (iii) at least one stabilizer comprising in its molecule at            least one group capable of inhibiting free-radical            polymerization, and at least one other group capable of            sorption at the hydrophilic surface of the substrate.

Unless defined otherwise, the terms “alkyl group, alkanediyl group oralkyl unit of an aralkyl group” as used in the present invention referto a straight-chain, branched or cyclic saturated hydrocarbon group,which optionally comprises one or more substituents selected fromhalogen atoms, C₁-C₁₂ alkyl,

(wherein R is selected from C₁-C₁₂ alkyl).

It is essential for the present invention that the radiation-sensitivecoating comprise one or more stabilizers, which comprise two kinds offunctional groups in one molecule:

-   1. a functional group capable of inhibiting free-radical    polymerization (in the following also referred to simply as    “inhibitor group”) and-   2. a functional group capable of sorption at the hydrophilic surface    of the substrate (in the following also referred to simply as    “sorption group”).

Within the framework of the present invention, the term “sorption”encompasses absorption, adsorption and chemisorption.

The stabilizer can be a monomeric or polymeric compound.

The inhibitor group is preferably derived from stable free radicals suchas e.g. nitroxyl radicals, mono-, di- or trihydroxy aromatics (e.g.derived from hydroquinone, hydroquinone monomethylester and phenols withsterically superior substituents in the 2- or 2- and 6-positions),quinones (e.g. derived from 1,2- or 1,4-benzoquinone), nitroso compounds(e.g. derived from nitrosobenzene, nitrosodurene andnitroso-tert.-butane) and mixtures thereof.

An especially preferred inhibitor group is represented by the followingformula:

wherein each A is independently selected from C₁-C₁₀ alkyl (preferablyC₁-C₃); the alkyl group can optionally be substituted with one or more(preferably one) substituents selected from halogen atoms or C₁-C₁₀alkoxy.

The nitroxyl function is part of a 5- or 6-membered heterocyclic ring,which optionally comprises one or more (preferably no or one) doublebonds and optionally contains, in addition to the nitrogen atom of thenitroxyl group, one or more (preferably one) further heteroatomsselected from O, S, and N (preferably O or N).

In an especially preferred embodiment the preferred inhibitor group offormula (I) is selected from:

The sorption group is preferably selected from P—OH-containing groups;especially preferred are:

wherein R¹ is selected from C₁-C₁₈ alkyl, C₇-C₁₁ aralkyl and C₆-C₁₀ aryland mixtures thereof, and X represents a single bond, —O—, —NH or—N(C₁-C₁₀ alkyl)-.

R¹ is preferably C₁-C₆ alkyl, C₇ aralkyl or C₆ aryl; preferably, X is—O—.

According to a preferred embodiment, the sorption group is

If the stabilizer is a monomeric compound, it is preferably selectedfrom compounds of formula (M1) and mixtures thereof.

wherein X is as defined above, R² represents hydrogen, C₁-C₁₈ alkyl,C₇-C₁₈ aralkyl, C₆-C₁₀ aryl or

each group A is as defined above and the nitroxyl function is part of a5- or 6-membered heterocyclic ring, which optionally comprises one ormore (preferably no or one) double bonds and optionally contains, inaddition to the nitrogen atom of the nitroxyl group, one or more furtherheteroatoms selected from O, S, and N (preferably no additionalheteroatom or one selected from O or N). If R² represents a group offormula (I), it is preferably selected from formulas (Ia) to (If) asshown above in connection with the inhibitor group.

In a stabilizer according to formula (M1), R² is preferably C₁-C₆ alkyl,C₇ aralkyl or C₆-aryl or a hydrogen atom, especially preferred, R² is ahydrogen atom. With respect to A, it is preferred that all groups A beidentical, preferably methyl groups.

However, the stabilizer can also be a polymeric compound, wherein atleast one sorption group and at least one inhibitor group are bonded toits polymeric parent structure.

A polymeric stabilizer is preferable a copolymer obtained bypolymerization of at least one of the following monomers: Fumaric acid,maleic acid anhydride (preferably decyclized with C₁-C₁₀ alcohols),ethylene, propylene, acrylic acid esters (preferably C₁-C₁₀ alkylester), methacrylic acid esters (preferably C₁-C₁₀ alkyl ester), vinylesters (preferably vinyl acetate), vinyl alcohol and styrene with atleast one copolymerizable monomer substituted with a sorption group andat least one polymerizable monomer substituted with an inhibitor group.

The structural unit of the polymeric stabilizer carrying the sorptiongroup preferably comprises P—OH-containing groups; the followingstructural units are especially preferred:

wherein X² is C₁-C₁₂ alkanediyl or C₆-C₁₀ arylene, R³ is C₁-C₁₂ alky, X¹is selected from —O—, —NH— and —N(C₁-C₁₀ alkyl) and R² is as definedabove.

X² is preferably C₂-C₆ alkanediyl.

R³ is preferably —CH₃.

X¹ preferably represents —O—.

R² is preferably hydrogen, C₁-C₆ alkyl, C₇ aralkyl and C₆ aryl,especially preferred hydrogen.

If X¹ is —N(C₁-C₁₀ alkyl), the alkyl group preferably comprises 1 to 4carbon atoms.

In a preferred embodiment, the sorption group of the polymericstabilizer is represented by the formula PSA₁.

The structural unit of the polymeric stabilizer carrying the inhibitorgroup is preferably selected from:

wherein A is defined as above with respect to a monomeric stabilizer, X¹and R³ are as defined above with respect to the sorption group, and thenitroxyl function is part of a 5- or 6-membered heterocyclic ring, whichoptionally comprises one or more double bonds and optionally contains,in addition to the nitrogen atom of the nitroxyl group, one or morefurther heteroatoms selected from O, S, and N.

In a preferred embodiment, the inhibitor group of the polymericstabilizer is represented by the formula PI₁.

It is preferred that at least 80 wt.-% of the stabilizer (iii) sorb atthe hydrophilic substrate surface, especially preferred at least 90wt.-%, and most preferred at least 95 wt.-%.

According to a preferred embodiment, the sorption is an absorption or achemisorption.

Spectroscopic testing indicates that the stabilizer is sorbed at thesubstrate surface; preferably, at least 80 wt.-% of the stabilizer aresorbed at the hydrophilic substrate surface, especially preferred atleast 90 wt.-%, and most preferred more than 95 wt.-%.

No concentration of the stabilizer at the surface of theradiation-sensitive coating or migration of the stabilizer to anoptionally present oxygen-impermeable overcoat takes place, which isconfirmed in spectroscopic tests.

All monomers, oligomers and polymers with C—C double bonds which arefree-radical polymerizable and comprise at least one C—C double can beused as ethylenically unsaturated monomers, oligomers and polymers.Monomers, oligomers and polymers with C—C triple bonds can also be used,but they are not preferred. Suitable compounds are well known to theperson skilled in the art and can be used in the present inventionwithout any particular limitations. Esters of acrylic and methacrylicacids, itaconic acid, crotonic acid, isocrotonic acid, maleic acid andfumaric acid with one or more unsaturated groups in the form ofmonomers, oligomers or prepolymers are preferred. They may be present insolid or liquid form, with solid and highly viscous forms beingpreferred. Compounds suitable as monomers include for instancetrimethylol propane triacrylate and methacrylate, pentaerythritetriacrylate and methacrylate, dipentaerythritemonohydroxy pentaacrylateand methacrylate, dipentaerythrite hexaacrylate and methacrylate,pentaerythrite tetraacrylate and methacrylate, ditrimethylol propanetetraacrylate and methacrylate, diethyleneglycol diacrylate andmethacrylate, triethyleneglycol diacrylate and methacrylate ortetraethyleneglycol diacrylate and methacrylate. Suitable oligomersand/or prepolymers are for example urethane acrylates and methacrylates,epoxide acrylates and methacrylates, polyester acrylates andmethacrylates, polyether acrylates and methacrylates or unsaturatedpolyester resins.

In addition to monomers and/or oligomers, use can also be made ofpolymers comprising free-radical polymerizable C—C double bonds in themain or side chains. Examples thereof include reaction products ofmaleic acid anhydride olefin copolymers and hydroxyalkyl(meth)acrylates;polyesters comprising an allylalcoholester group; reaction products ofpolymeric polyalcohols and isocyanato(meth)acrylates; unsaturatedpolyesters; (meth)acrylate-terminated polystyrenes, poly(meth)acrylicacid ester, poly(meth)acrylic acids, poly(meth)acrylamides andpolyethers. In this connection, the prefix “(meth)” indicates that bothderivatives of acrylic acid and of methacrylic acid can be used.

Additional suitable C—C unsaturated free-radical polymerizable compoundsare described e.g. in EP-A-1 176 007.

It is of course possible to use different kinds of monomers, oligomersor polymers in the mixture; furthermore, mixtures of monomers andoligomers and/or polymers can be used, as well as mixtures of oligomersand polymers. The free-radical polymerizable monomers, oligomers orpolymers are preferably present in an amount of 35 to 90 wt.-%; ifmonomers/oligomers are used, 45 to 60 wt.-%, based on the dry layerweight of the IR-sensitive coating, are especially preferred.

In the present invention, a photoinitiator is a compound capable ofabsorbing radiation when exposed and of forming free radicals by itself,i.e. without the addition of coinitiators. Examples of suitablephotoinitiators include triazine derivatives with 1 to 3 CX₃ groups(wherein every X is independently selected from a chlorine or bromineatom, and is preferably a chlorine atom), hexaarylbisimidazolecompounds, benzoin ethers, benzyl ketals, oxime ethers, oxime esters,α-hydroxy- or α-amino-acetophenones, acylphosphines, acylphosphineoxides, acylphosphine sulfides, metallocenes, peroxides etc. Examples ofsuitable triazine derivatives include2-phenyl-4,6-bis(trichloromethyl)-s-triazine,2,4,6-tris(trichloromethyl)-s-triazine,2-methyl-4,6-bis(trichloromethyl)-s-triazine,2-(styryl-4,6-bis(trichloromethyl)-s-triazine,2-(p-methoxystyryl)-4,6-bis(trichloromethyl)-s-triazine,2-(4-methoxy-naphtho-1-yl)-4,6-bis-trichloromethyl-s-triazine, and2-(4-ethoxy-naphtho-1-yl)-4,6-bis(trichloromethyl)-s-triazine and2-[4-(2-ethoxyethyl)-naphtho-1-yl]-4,6-bis(trichloro-methyl)-s-triazine. Suitable oxime ethers and oxime estersare for example those derived from benzoin. Preferred metallocenes arefor example titanocenes with two five-membered cyclodienyl groups suchas e.g. cyclopentadienyl groups and one or two six-membered aromaticgroups having at least one ortho-fluorine atom and optionally also onepyrryl group; most preferred metallocenes arebis(cyclopentadienyl)-bis-[2,6-difluoro-3-(pyrr-1-yl)-phenyl]titaniumand dicyclopentadiene-bis-2,4,6-trifluorophenyl-titanium or zirconium.

In the present invention, a single photoinitiator or a mixture of two ormore can be used.

The photoinitiators can be used alone or in combination with one or morecoinitiators; the addition of coinitiators can increase theeffectiveness of the photoinitiation.

The amount of photoinitiator(s) is not particularly restricted; however,if photoinitiators are present, it is preferably in the range of 0.2 to25 wt.-%, based on the dry layer weight, especially preferred 0.5 to 15wt.-%.

A sensitizer as referred to in the present invention is a compound whichcan absorb radiation when it is exposed but which cannot by itself, i.e.without the addition of coinitiators, form free radicals.

All light-absorbing compounds that are photooxidizable or photoreducibleor capable of transferring their excitation energy to receptor moleculesare suitable sensitizers for use in the present invention. Examples ofsuch dyes include cyanine dyes, merocyanine dyes, oxonol dyes,diarylmethane dyes, triarylmethane dyes, xanthene dyes, coumarinderivatives, ketocoumarin dyes, acridine dyes, phenazine dyes,quinoxaline dyes, pyrrylium dyes or thiapyrrylium dyes, azaanulene dyes(such as phthalocyanines and porphyrines), indigo dyes, anthraquinonedyes, polyarylenes, polyarylpolyenes, 2,5-diphenylisobenzofuranes,2,5-diarylfuranes, 2,5-diarylthiofuranes, 2,5-diarylpyrroles,2,5-diarylcyclopentadienes, polyarylphenylenes, polyaryl-2-pyrazolines,carbonyl compounds such as aromatic ketones or quinones, e.g.benzophenone derivatives, Michler's ketone, thioxanthone derivatives,anthraquinone derivatives and fluorenone derivatives.

In the present invention, one sensitizer or a mixture of two or more canbe used.

The sensitizers are used in combination with one or more coinitiators.Additionally, photoinitiators can be used; however, this is notpreferred.

The amount of sensitizer(s) is not particularly restricted; however, ifsensitizers are present, it is preferably in the range of 0.2 to 15wt.-%, based on the dry layer weight, especially preferred 0.5 to 10wt.-%. If both photoinitiators and sensitizers are present in thecoating, their total amount is preferably 0.5 to 30 wt.-%, based on thedry layer weight, especially preferred 1 to 15 wt.-%.

A coinitiator as referred to in the present invention is a compound thatis essentially unable to absorb when exposed and forms free radicalstogether with the light-absorbing sensitizers. Coinitiators suitable foruse in the present invention include e.g. amines such as alkanol aminesor 4-dialkylaminebenzoic acid esters, diaryliodonium salts,triarylsulforium salts, aryldiazonium salts, N-arylglycine derivatives,diaryldiiminodiacetic acid compounds, aryloxyacetic acid, aromaticsulfonyl halides, trihalomethylsulfones, nitrogen heterocycles withN-oxide substituents (such as alkoxy pyridinium salts), nitrogenheterocycles such as N-benzoyloxyphthalimide, diazosulfonates,9,10-dihydroanthracene derivatives, thiol compounds (e.g.mercaptobenzthiazole, mercaptobenzimidazole and mercaptotriazole),triazine derivatives with 1 to 3 CX₃ groups (wherein every X isindependently selected from a chlorine or bromine atom, and ispreferably a chlorine atom), hexaarylbisimidazole compounds, benzoinethers, benzyl ketals, oxine ethers, oxime esters, α-hydroxy- orα-amino-acetophenones, acylphosphines, acylphosphine oxides,acylphosphine sulfides, metallocenes, peroxides etc., carbonyl compoundssuch as aromatic ketones or quinones, e.g. benzophenone derivatives,Michler's ketone, thioxanthone derivatives, anthraquinone derivativesand fluorenone derivatives.

Many coinitiators can also function as photoinitiators when they areexposed in their absorption band. This way, photosensitive layers can beobtained that are e.g. sensitive over a wide spectral range because aphotoinitiator or sensitizer covers the long-wavelength spectral range(IR and/or visible range) and a coinitiator covers the short-wavelengthspectral range (e.g. the UV range). This effect can be advantageous ifthe consumer wants to irradiate the same material with differentradiation sources. For example, the coinitiator can function as anactual coinitiator in the sense of the definition given above for the IRor visible range, while it functions as a photoinitiator for the UVrange.

In the present invention, one coinitiator or a mixture of coinitiatorscan be used.

The amount of coinitiator(s) is not particularly restricted; however, itis preferably in the range of 0.2 to 25 wt.-%, based on the dry layerweight, especially preferred 0.5 to 15 wt.-%.

Optionally, the radiation-sensitive coating of the present invention canalso comprise a binder or a mixture of binders. The binder is preferablyselected from polyvinyl acetals, acrylic polymers and polyurethanes. Itis preferred that the binder contains acid groups, especially preferredcarboxy groups. Most preferred are acrylic polymers. Binders with acidgroups preferably have acid numbers in the range of 20 to 180 mg KOH/gpolymer. Optionally, the binder can comprise unsaturated groups in themain chain or the side chains. Such unsaturated bonds are capable ofundergoing a free-radical photopolymerization reaction or anotherphotoreaction such as e.g. a 2+2-photocycloaddition.

The radiation-sensitive coating can optionally also comprise smallamounts of a thermopolymerization inhibitor which is not suitable forsorption at the hydrophilic surface of the substrate. Suitable examplesof polymerization inhibitors include hydroquinone, p-methoxyphenol,di-t-butyl-p-cresol, pyrrogallol, t-butylcatechol, benzoquinone,4,4′-thio-bis-(3-methyl-6-t-butylphenol),2,2′-methylene-bis-(4-methyl-6-t-butylphenol) andN-nitroso-phenylhydroxylamine salts. The amount of the non-absorbablepolymerization inhibitor in the radiation-sensitive coating ispreferably 0 to 5 wt.-%, based on the dry layer weight, especiallypreferred 0.01 to 2 wt.-%. Such inhibitors are often introduced into theradiation-sensitive coating via commercial monomers or oligomers and aretherefore not expressly mentioned.

Furthermore, the radiation-sensitive coating of the present inventioncan comprise dyes or pigments for coloring the layer. Examples ofcolorants include e.g. phthalocyanine pigments, azo pigments, carbonblack and titanium dioxide, ethyl violet, crystal violet, azo dyes,anthraquinone dyes and cyanine dyes. The amount of colorant ispreferably 0 to 20 wt.-%, based on the dry layer weight, especiallypreferred 0.5 to 10 wt.-%.

For improving the physical properties of the hardened layer, theradiation-sensitive coating can additionally comprise further additivessuch as plasticizers or inorganic fillers. Suitable plasticizers includee.g. dibutyl phthalate, dioctyl phthalate, didodecyl phthalate, dioctyladipate, dibutyl sebacate, triacetyl glycerin and tricresyl phosphate.The amount of plasticizer is not particularly restricted, however, it ispreferably 0 to 10 wt.-%, based on the dry-layer weight, especiallypreferred 0.25 to 5 wt.-%.

The radiation-sensitive coating can also comprise known chain transferagents such as e.g. mercapto compounds. They are preferably used in anamount of 0 to 15 wt.-%, based on the dry layer weight, especiallypreferred 0.5 to 5 wt.-%.

Furthermore, the radiation-sensitive coating can comprise leuco dyessuch as e.g. leuco crystal violet and leucomalachite green. They arepreferably present in an amount of 0 to 10 wt.-%, based on the dry layerweight, especially preferred 0.5 to 5 wt.-%.

Additionally, the radiation-sensitive coating can comprise surfactants.Suitable surfactants include siloxane-containing polymers,fluorine-containing polymers and polymers with ethylene oxide and/orpropylene oxide groups. They are preferably present in an amount of 0 to10 wt.-%, based on the dry layer weight, especially preferred 0.2 to 5wt.-%.

Further optional components of the radiation-sensitive coating includeinorganic fillers such as e.g. Al₂O₃ and SiO₂. They are preferablypresent in an amount of 0 to 20 wt.-%, based on the dry layer weight,especially preferred 0.1 to 5 wt.-%.

The radiation-sensitive elements of the present invention can forexample be printing plate precursors (in particular precursors oflithographic printing plates), printed circuit boards for integratedcircuits or photomasks.

A dimensionally stable plate or foil-shaped material is preferably usedas a substrate in the production of printing plate precursors.Preferably, a material is used as dimensionally stable plate orfoil-shaped material that has already been used as a substrate forprinting matters. Examples of such substrates include paper, papercoated with plastic materials (such as polyethylene, polypropylene,polystyrene), a metal plate or foil, such as e.g. aluminum (includingaluminum alloys), zinc and copper plates, plastic films made e.g. fromcellulose diacetate, cellulose triacetate, cellulose propionate,cellulose acetate, cellulose acetatebutyrate, cellulose nitrate,polyethylene terephthalate, polyethylene, polystyrene, polypropylene,polycarbonate and polyvinyl acetate, and a laminated material made frompaper or a plastic film and one of the above-mentioned metals, or apaper/plastic film that has been metallized by vapor deposition. Amongthese substrates, an aluminum plate or foil is especially preferredsince it shows a remarkable degree of dimensional stability; isinexpensive and furthermore exhibits excellent adhesion to the coating.Furthermore, a composite film can be used wherein an aluminum foil hasbeen laminated onto a polyethylene terephthalate film.

It is important in the present invention that the substrate has ahydrophilic surface on one side; substrates having a hydrophilic surfaceon their front and back sides can also be used in the present invention.The hydrophilic nature of the surface can be an inherent property of thesubstrate material itself, or be generated by treating the substratesurface with suitable substances known to the person skilled in the art.A substrate that exhibits a low degree of hydrophilia which isreinforced by the surface treatment is also encompassed by the presentinvention.

A metal substrate, in particular an aluminum substrate, is preferablysubjected to at least one treatment selected from roughening (e.g. bybrushing in a dry state or brushing with abrasive suspensions, orelectrochemical roughening, e.g. by means of a hydrochloric acidelectrolyte), anodic oxidation and application of a hydrophilizinglayer.

In order to improve the hydrophilic properties of the surface of themetal substrate that has been roughened and optionally anodicallyoxidized in sulfuric acid or phosphoric acid, the metal substrate can besubjected to an aftertreatment with an aqueous solution of sodiumsilicate, calcium zirconium fluoride, polyvinylphosphonic acid orphosphoric acid. Within the framework of the present invention, the term“substrate” also encompasses an optionally pre-treated substrateexhibiting, for example, a hydrophilizing layer on its surface.

The details of the above-mentioned substrate pre-treatment are known tothe person skilled in the art.

The radiation-sensitive coating of the present invention is applied tothe hydrophilic surface of the substrate by means of common coatingprocesses. If both the front and the back sides of the substrate have ahydrophilic surface, a radiation-sensitive coating can be applied onboth sides; in the elements according to the present invention it ispreferred that a radiation-sensitive coating is only applied to one sideof the substrate.

The radiation-sensitive elements of the present invention can beprepared as follows:

By means of known coating processes (e.g. centrifugal coating, dipcoating, coating by means of doctor blades), a radiation-sensitivemixture comprising

-   (i) at least one free-radical polymerizable monomer and/or oligomer    and/or polymer with at least one ethylenically unsaturated group    each,-   (ii) at least one absorber selected from polymerization initiators    and sensitizers, which is capable of absorbing radiation of a    wavelength in the range of 250 to 1,200 nm,-   (iii) at least one stabilizer comprising in its molecule at least    one group capable of sorption at the hydrophilic surface of the    substrate and at least one group capable of inhibiting free-radical    polymerization,-   (iv) a solvent or solvent mixture, and-   (v) optionally at least one additive selected from coinitiators,    which form free radicals after the excitation of the initiator or    sensitizer with radiation of a wavelength of 250 to 1,200 nm,    alkali-soluble binders, dyes, plasticizers, chain transfer agents,    leuco dyes, inorganic fillers, surfactants and polymerization    inhibitors not suitable for sorption at the surface of the substrate    is applied on the hydrophilic surface of the substrate.

Spectroscopic testing has shown that the stabilizer is concentrated atthe substrate surface; in a preferred embodiment, at least 80 wt.-% ofthe stabilizer are concentrated at the hydrophilic substrate surface,especially preferred at least 90 wt.-%, and most preferred more than 95wt.-%.

If both the front and the back sides of the substrate have a hydrophilicsurface, a radiation-sensitive coating can be applied on both sides, ifdesired.

Afterwards, the element is dried and an oxygen-impermeable overcoat isoptionally applied by means of known processes.

Suitable solvents include low alcohols (e.g. methanol, ethanol, propanoland butaniol), glycolether derivatives (e.g. ethyleneglycolmonomethylether, propyleneglycol mono-methylether, ethyleneglycolmonomethylether acetate and propyleneglycol monomethyl-ether acetate),ketones (e.g. methyl ethyl keton, methyl isobutyl keton), esters (e.g.ethyl acetate and butyl acetate), aromatics (e.g. toluene and xylene)and dipolar aprotic solvents (e.g. THF, dimethylformamide andN-methylpropyrrolidone). The solids content of the radiation-sensitivemixture to be applied depends on the coating method that is used and ispreferably 1 to 20 wt.-%.

Alternatively, the radiation-sensitive elements can be preparedaccording to the following two-step coating process:

In a first step, a mixture that is not radiation-sensitive and comprisesat least one solvent and at least one stabilizer comprising in itsmolecule at least one group capable of sorption at the hydrophilicsurface of the substrate and at least one group capable of inhibitingfree-radical polymerization is applied. The term “mixture that is notradiation-sensitive” refers to a mixture comprising neitherphotoinitiators nor sensitizers nor coinitiators.

After drying, a radiation-sensitive mixture comprising

-   (i) at least one free-radical polymerizable monomer and/or oligomer    and/or polymer with at least one ethylenically unsaturated group;-   (ii) at least one absorber selected from polymerization initiators    and sensitizers, which is capable of absorbing radiation of a    wavelength in the range of 250 to 1,200 nm;-   (iii) a solvent or solvent mixture, and-   (iv) optionally at least one additive selected from coinitiators,    which form free radicals after the excitation of the initiator or    sensitizer with radiation of a wavelength of 250 to 1,200 nm,    alkali-soluble binders, dyes, plasticizers, chain transfer agents,    leuco dyes, inorganic fillers, surfactants and polymerization    inhibitors not suitable for sorption at the surface of the substrate    is applied thereon in a second coating step.

In this two-step process as well, the substrate can optionally be coatedon both sides and an oxygen-impermeable overcoat can be applied on thedried radiation-sensitive coating.

Spectroscopic testing has shown that the stabilizer is not homogeneouslydispersed in the radiation-sensitive mixture when it is applied;preferably, at least 80 wt.-% of the stabilizer used remains on thesubstrate surface, especially preferred at least 90 wt.-%, and mostpreferred more than 95 wt.-%.

The radiation-sensitive coating of the radiation-sensitive elementsaccording to the present invention can consist of several “layers”. Itis not necessary for all components of the radiation-sensitive coatingto be uniformly dispersed throughout the entire thickness of theradiation-sensitive coating; for instance, the stabilizer sorbed at thesubstrate surface is also considered part of the radiation-sensitivecoating.

The additional application of a water-soluble oxygen-impermeableovercoat onto the radiation-sensitive layer can be advantageous. Thepolymers suitable for such an overcoat include, inter alia, polyvinylalcohol, polyvinyl alcohol/polyvinyl acetate copolymers, polyvinylpyrrolidone, polyvinyl pyrrolidone/polyvinyl acetate copolymers andgelatin. Preferably, the composition for the oxygen-impermeable overcoatis applied in the form of a solution in water or in a solvent misciblewith water; in any case, the solvent is selected such that theradiation-sensitive coating already present on the substrate does notdissolve. The layer weight of the overcoat can e.g. be 0.1 to 4 g/m²,preferably 0.3 to 3 g/m². However, the printing plate precursorsaccording to the present invention show excellent properties evenwithout an overcoat. The overcoat can also comprise matting agents (i.e.organic or inorganic particles with a particle size of 2 to 20 μm) whichfacilitate the planar positioning of the film during contact exposure.If an overcoat is present, it is essentially free of the stabilizersused in the present invention. The overcoat according to the presentinvention is permeable for radiation; essentially no photochemicalreactions take place in this layer, which also distinguishes this layerfrom the radiation-sensitive coating.

The thus produced printing plate precursors are image-wise exposed in amanner known to the person skilled in the art with radiation of anappropriate wavelength and subsequently developed with a commerciallyavailable aqueous alkaline developer. After image-wise exposure, i.e.prior to developing, a heat treatment at 50 to 180° C., preferably 90 to150° C., can be carried out. The developed plates can be treated with apreservative (“gumming”) using a common method. The preservatives areaqueous solutions of hydrophilic polymers, wetting agents and otheradditives.

For certain applications, it is furthermore advantageous to increase themechanical strength of the printing layers by subjecting them to a heattreatment (what is referred as “baking”) and/or a combination of bakingand overall exposure (e.g. to UV light). For this purpose, prior to thetreatment, the plate is treated with a solution that protects thenon-image areas such that the heat treatment does not cause these areasto accept ink. A solution suitable for this purpose is e.g. described inU.S. Pat. No. 4,355,096. Baking takes place at a temperature in therange of 150 to 250° C. However, printing plates prepared from printingplate precursors according to the present invention show excellentproperties even without having been subjected to a heat treatment. Whenboth baking and overall exposure are carried out, the two treatmentsteps can be performed simultaneously or one after the other.

Lithographic printing plate precursors according to the presentinvention are characterized by improved photosensitivity in combinationwith good storage stability; the developed printing plates exhibitexcellent abrasion resistance which allows a large number of copies.

The invention will be explained in more detail in the followingexamples.

EXAMPLES Preparation Example 1

13.54 g vinylphosphonic acid (0.125 mole), 12.26 g maleic acid anhydride(0.125 mole), 25.03 g methylmethacrylate (0.25 mole) and 0.25 g AIBN(0.0015 mole) were dissolved in 65 g methyl ethyl ketone. The solutionwas stirred and heated in a nitrogen atmosphere for 8 hours. The solidscontent of the solution was 38.2 wt.-%. 0.35 g 4-amino-TEMPO were addedto 2.2.8 g of this solution and the solution was heated to 60° C. forthree hours under stirring. Then 0.0045 g pure methylamine (acorresponding amount of a 2 molar solution of TMF was used) were addedand the solution was held at a temperature of 60° C. for another hour.The solution was stored in a refrigerator until it was used.

Examples 1 to 4 and Comparative Examples 1 to 12

An electrochemically roughened and anodized aluminum foil was subjectedto a treatment with an aqueous solution of polyvinyl phosphonic acid(PVPA) and was coated, after drying, with an aqueous solution asdescribed in Table 1 and dried; a lithographic printing plate precursorwas obtained.

TABLE 1 Coating solution 0.82 g of a terpolymer prepared by polymerizing476 parts by weight styrene, 476 parts by weight methylmetha- crylateand 106 parts by weight methacrylic acid 0.1 g Kayamer PM-2 (1 molephosphoric acid esterified with 1.5 moles hydroxyethylmethacrylate fromCoa Corp. Ltd., Japan) 0.2 g mercapto-3-triazole 3.58 g of an 80% methylethyl ketone solution of a urethane acrylate prepared by reactingDesmodur N100 ® (available from the company Bayer) with hydroxy ethylacrylate and pentaerythritol triacrylate; double-bond content: 0.50double bonds/100 g when all isocyanate groups have reacted with theacrylates comprising hydroxyl groups 0.41 g ditrimethylolpropanetetraacrylate 1.25 g of a dispersion in propyleneglycol monomethylethercomprising 7.25 wt-% copper phthalocyanine and 7.25 wt.-%polyvinylacetal binder comprising 39.9 mole-% vinyl alcohol groups, 1.2mole-% vinyl acetate groups, 15.4 mole-% acetal groups derived fromacetaldehyde, 36.1 mole-% acetal groups derived from butyric aldehydeand 7.4 mole-% acetal groups derived from 4-formylbenzoic acid 0.115 gbis(cyclopentadienyl)-bis-[2,6-difluoro-3- (pyrr-1-yl)-phenyl]titanium0.16 g diphenyliodonium chloride 0.15 g ethyleosin X g additiveaccording to Table 2 45 ml propyleneglycol monomethylether 5 ml acetone

The solution was filtered, applied to the pretreated substrate and thecoating was dried for 5 minutes at 90° C. The dry layer weight of thephotopolymer layer was about 2 g/m².

The obtained samples were coated with an overcoat by applying an aqueoussolution of poly(vinylalcohol) (Airvol 203 available from Airproducts;degree of hydrolysis: 88%). After drying for 5 minutes at 90° C., theovercoat had a dry layer weight of about 3 g/m².

The printing plate precursor was exposed to the light of a tungsten lamphaving a metal interference filter for 532 nm for 60 seconds through agray scale having a tonal range of 0.15 to 1.95, wherein the densityincrements amount to 0.15 (UGRA gray scale). Immediately after exposure,the plate was heated in an oven for 2 minutes at 90° C.

Then, the exposed plate was treated for 30 seconds with a developersolution comprising the following components:

3.4 parts by weight Rewopol NLS 28® (available from the company REWO)

1.1 parts by weight diethanol amine

1.0 parts by weight Texapon 842® (available from the company Henkel)

0.6 parts by weight Nekal BX Paste® (available from the company BASF)

0.2 parts by weight 4-toluene sulfonic acid and

93.7 parts by weight water

Then the developer solution was again rubbed over the surface foranother 30 seconds using a tampon and then the entire plate was rinsedwith water. After this treatment, the exposed portions remained on theplate. For the assessment of its photosensitivity, the plate wasblackened in a wet state with printing ink.

For the assessment of storage stability, the unexposed printing plateprecursors were stored for 60 minutes in a 90° C. oven, then exposed anddeveloped as described above (storage stability test).

For evaluating the latitude during preheating, the exposed plates wereheated in an infrared heating device NE459/125P from BasysPrint at aplate rate of 60 cm/min, and the temperature on the plate surface,measured with a temperature strip on the back of the plate, was 140° C.(preheating latitude test).

For the preparation of a lithographic printing plate, a printing layerwas applied to the aluminum foil, as explained above, exposed, heated,developed, and after rinsing with water, the developed plate was rubbedwith an aqueous solution of 0.5% phosphoric acid and 6% gum arabic. Thethus prepared plate was loaded in a sheet-fed offset printing machineand used for printing with an abrasive printing ink (Offset S 7184available from Sun Chemical, containing 10% potassium carbonate).

The results are summarized in Table 2.

In order to illustrate the sorption of the stabilizer on the surface ofthe lithographic plate, the following experiments were carried out(Table 3):

-   -   The photopolymer layer was applied on the lithographic substrate        (Al foil) such that a dry layer weight of 2 g/m², as described        above, was obtained; the compositions of the samples were the        same as described in Table 1 and Table 2. An oxygen-impermeable        overcoat with a dry layer weight of 3 g/m² was applied on the        photopolymer layer.    -   The overcoat was washed with H₂O.    -   The photopolymer layer on the Al foil was removed with methyl        ethyl ketone (20 ml methyl ethyl ketone per 500 cm²).    -   The photopolymer layer was applied onto a polyester film and a        dry layer weight of 2 g/m² was obtained. As in the case of the        Al foil, an oxygen-impermeable overcoat with a dry layer weight        of 3 g/m² was applied here as well;    -   The overcoat was washed.    -   The photopolymer layer on the polyester film was removed with        methyl ethyl ketone (20 ml methyl ethyl ketone per 500 cm²).    -   EPR spectra were recorded of the two samples of removed        photopolymer layer (one removed from the Al foil and the second        one removed from the polyester film) and the signal intensities        were compared. The resulting EPR spectra showed a typical        interpolation spectrum with hyperfine splitting of about 13.7        Gauss and a g-value of about 2.0051. By integrating the signal        intensity of the sample of the photopolymer layer removed from        the lithographic substrate (S1) and the sample of the        photopolymer layer removed from the polyester film (S2) the        amount of nitroxyl radicals was calculated. The value of the        polyester sample from which the photopolymer layer had been        removed was used as a reference and was determined to be 100%.        The percentage of sorbed nitroxyl radicals was calculated as        follows:        R%=100%−(S1/S2)*100%

The results are summarized in Table 3.

The results summarized in Table 3 clearly show that the stabilizersaccording to the present invention were sorbed at the surface of thelithographic plate to a degree of almost 100%. The nitroxyl radicalcompounds of Comparative Examples 1 to 8 and 11 to 14 showed practicallyno interaction with the surface of the lithographic substrate.

Furthermore, it can be inferred from Table 2 that the use of thestabilizers improved the storage stability and the heating latitudewithout essentially affecting the sensitivity. Although stabilizerswithout functional groups capable of sorption at the substrate surfaceimprove the storage stability and the heating latitude in some cases,they considerably impair sensitivity.

In conclusion, both the inhibitor group and the sorption groups areessential for an additive leading to improved storage stability and alarger heating latitude without affecting the photosensitivity of theplate.

TABLE 2 Stabilizer applied Gray Gray scale Stabilizer applied as betweenlithographic scale Gray scale 532 nm¹⁾ component of the radiation-substrate and radiation- 532 532 nm¹⁾ 140° C./60 cm Example sensitivemixture⁸⁾ sensitive layer⁸⁾ nm¹⁾ 60 min/90°⁶⁾ min⁷⁾ Printing resultsComp. 1 none none 3/8 3/7 not developable abrasion visible after 170,000copies to 9 1 0.00005 g TEMPO phosphate none 3/8 3/8 3/8 abrasionvisible after 170,000 copies 0.00010 g TEMPO phosphate none 3/8 3/8 3/8not determined 0.00015 g TEMPO phosphate none 3/8 3/8 3/8 abrasionvisible after 160,000 copies 2 0.0005 g copolymer acc. to none 3/8 3/83/8 abrasion visible after 160,000 copies Preparation Example 1 3 none0.0006 g/m² 3/8 3/8 3/8 abrasion visible after 180,000 copies TEMPOphosphate to 9 4 none 0.0012 g/m² copolymer 3/8 3/8 3/8 abrasion visibleafter 170,000 copies according to Preparation Example 1 Comp. 2 0.0015 gOH TEMPO none 2/8 1/5 to 6 1/5 to 6 abrasion visible after 130,000copies Comp. 3 none 0.0006 g/m² 2/6 2/6 3/6 abrasion visible after140,000 copies OH-TEMPO Comp. 4 0.0025 g TEMPO none 2/6 2/5 notdevelopable not determined Comp. 5 0.0025 g TEMPO none 2/6 2/5 notdevelopable abrasion visible after 120,000 copies phosphate ester Comp.6 0.0006 g carboxy-proxyl none 2/8 2/6 not developable not determined0.00125 g carboxy-proxyl none 2/6 2/6 1/6 abrasion visible after 100,000copies Comp. 7 0.00005 g NH₂ TEMPO none 2/7 2/6 not developable notdetermined 0.0001 g NH₂ TEMPO none 2/6 2/5 1/4 not determined Comp. 80.000125 g CGS-160 none 2/6 1/4 not developable not determined 0.0005 gCGS-160 none 1/3 1/2 1/2 not determined Comp. 9 0.0001 g 4-methoxyphenolnone 3/6 3/6 not developable abrasion visible after 120,000 copies Comp.10 0.005 g Irganox 1010⁴⁾ none 2/6 2/6 not developable abrasion visibleafter 120,000 copies Comp. 11 0.00025 g polymer with none 2/6 2/5 notdevelopable abrasion visible after 150,000 copies TEMPO groups accordingto WO 96/34314²⁾ 0.0005 g polymer with none 2/4 2/4 2/3 abrasion visibleafter 140,000 copies TEMPO groups according to WO 96/34314²⁾ Comp. 12 ³⁾none 3/6 3/6 not developable abrasion visible after 140,000 copies Comp.13 Q-1301 none 2/5 2/5 not developable abrasion visible after 110,000copies Comp. 14 0.050 g phosphoric acid none 3/6 2/5 not developable notdetermined Footnotes for Table 2 ¹⁾The first value indicates the solidsteps of the blackened gray scale and the second value indicates thefirst step that did not accept printing ink. ²⁾The polymer was preparedaccording to WO 96/34314 by reacting an maleic acid anhydride ethylenecopolymer with 4-amino TEMPO and taurine ³⁾0.05% of the polymerdescribed in ²⁾ above were present in the overcoat. ⁴⁾Irganox 1010 is acommercially available inhibitor of the company CIBASpecialities/Switzerland; it ispentaerythritol-tetrakis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate).⁵⁾Q-1301 is a commercially available inhibitor of the company WakoChemicals Ltd./Japan; it is N-nitrosophenylhydroxylamine-aluminum salt.⁶⁾Storage stability: Result obtained after 60 minute storage of theunexposed plate precursor at 90° C. ⁷⁾Preheating latitude test: Resultobtained after the exposed plate passed through a 140° C. oven. ⁸⁾Theabbreviations of the additives have the following meaning:

TABLE 3 Additive Y applied between EPR Additive X lithographic Test inthe coating substrate and No. solution photopolymer layer R % 1  nonenone no signal obtained 2  0.0025 TEMPO none   98%  phosphate 3  0.0075g copolymer none   95%  according to Preparation Example 1 4  none 0.001g/m² 100%   TEMPO phosphate 5* 0.0025 g OH TEMPO none <5% 6* 0.0025TEMPO none <5% 7* 0.0025 g TEMPO none <5% phosphate ester 8* 0.00125 gnone <5% carboxy-proxyl none <5% 0.0025 carboxy-proxyl 9* 0.0025 g NH₂TEMPO none <5% 10*  0.00125 g CGS-169 none <5% 0.005 g CGS-169 none <5%11*  0.0075 g polymer none <5% with TEMPO groups according to WO96/34314²⁾ *Comparison

1. A radiation-sensitive element comprising (a) a substrate with atleast one hydrophilic surface and (b) a radiation-sensitive coating onat least one hydrophilic surface of the substrate, wherein the coatingcomprises: (i) at least one free-radical polymerizable monomer,oligomer, polymer or mixture thereof with at least one ethylenicallyunsaturated group each, (ii) at least one photoinitiator or sensitizer,which is capable of absorbing radiation of a wavelength in the range of250 to 1,200 nm, (iii) at least one stabilizer compound comprising atleast one group capable of inhibiting free-radical polymerization, andat least one other group capable of sorption at the hydrophilic surfaceof the substrate, wherein the stabilizer compound is a monomericcompound, and (iv) optionally at least one additive comprisingcoinitiators which form free radicals after the excitation of thephotoinitiator or sensitizer with radiation of a wavelength of 250 to1,200 nm, binders, dyes, plasticizers, chain transfer agents, leucodyes, inorganic fillers, surfactants or polymerization inhibitors notsuitable for sorption at the surface of the substrate, wherein the atleast one group of the stabilizer compound capable of sorption at thesurface of the substrate is:

wherein R¹ is C₁-C₁₈ aralkyl or C₆-C₁₀ aryl and X represents a singlebond, —O—, —NH— or —N(C₁-C₁₀ alkyl)-.
 2. The radiation-sensitive elementaccording to claim 1, wherein the monomeric stabilizer compound isrepresented by formula M1:

wherein X represents a single bond, —O—, —NH— or —N(C₁-C₁₀ alkyl)-, R²is hydrogen, C₁-C₁₈ alkyl, C₇-C₁₁ aralkyl, C₆-C₁₀ aryl or a fragment

wherein each A is independently optionally substituted C₁-C₁₀ alkyl andthe nitroxyl function is part of a 5- or 6-membered heterocyclic ring,which optionally comprises one or more double bonds and optionallycontains, in addition to the nitrogen atom of the nitroxyl group, one ormore O, S or N heteroatoms.
 3. The radiation-sensitive element accordingto claim 2, wherein the fragment

is represented by formulas Ia-If


4. The radiation-sensitive element according to claim 2, wherein Xrepresents —O—.
 5. The radiation-sensitive element according to claim 2,wherein R² is a hydrogen atom.
 6. The radiation-sensitive elementaccording to claim 2, wherein A is methyl.
 7. A radiation-sensitiveelement comprising (a) a substrate with at least one hydrophilic surfaceand (b) a radiation-sensitive coating on at least one hydrophilicsurface of the substrate, wherein the coating comprises: (i) at leastone free-radical polymerizable monomer, oligomer, polymer or mixturethereof with at least one ethylenically unsaturated group each, (ii) atleast one photoinitiator or sensitizer, which is capable of absorbingradiation of a wavelength in the range of 250 to 1,200 nm, (iii) atleast one stabilizer compound comprising at least one group capable ofinhibiting free-radical polymerization, and at least one other groupcapable of sorption at the hydrophilic surface of the substrate, and(iv) optionally at least one additive comprising coinitiators which formfree radicals after the excitation of the photoinitiator or sensitizerwith radiation of a wavelength of 250 to 1,200 nm, binders, dyes,plasticizers, chain transfer agents, leuco dyes, inorganic fillers,surfactants or polymerization inhibitors not suitable for sorption atthe surface of the substrate, wherein the element further comprises anoxygen-impermeable overcoat.
 8. A process for the production of aradiation-sensitive element comprising the steps of: (a) providing anoptionally pretreated substrate having a hydrophilic surface; (b)applying to said optionally pretreated substrate having a hydrophilicsurface, a radiation-sensitive mixture comprising (i) at least onefree-radical polymerizable monomer, oligomer, polymer or mixture thereofwith at least one ethylenically unsaturated group each, (ii) at leastone photoinitiator or sensitizer, which is capable of absorbingradiation of a wavelength in the range of 250 to 1,200 nm, (iii) atleast one stabilizer compound comprising at least one group capable ofsorption at the hydrophilic surface of the substrate, and at least oneother group capable of inhibiting free-radical polymerization, whereinthe at least one group of the stabilizer compound capable of sorption atthe surface of the substrate is:

 wherein R¹ is C₁-C₁₈ alkyl, C₇-C₁₁ aralkyl or C₆-C₁₀ aryl and Xrepresents a single bond, —O—, —NH— or —N(C₁-C₁₀ alkyl)-, (iv) a solventor solvent mixture, and (v) optionally at least one additive comprisingcoinitiators which form free radicals after the excitation of thephotoinitiator or sensitizer with radiation of a wavelength of 250 to1,200 nm, binders, dyes, plasticizers, chain transfer agents, leucodyes, inorganic fillers, surfactants or polymerization inhibitors notsuitable for sorption at the surface of the substrate; and c) drying thelayer obtained in step (b).
 9. The process according to claim 8, whereinthe process further comprises the step of applying an oxygen-impermeableovercoat to the layer dried in step (c).
 10. A process for theproduction of a radiation-sensitive element comprising the steps of: (a)providing an optionally pretreated substrate having a hydrophilicsurface; (b) applying to said optionally pretreated substrate having ahydrophilic surface, a mixture that is not radiation-sensitivecomprising at least one solvent and at least one stabilizer compoundcomprising at least one group capable of sorption at the substrate andat least one group capable of inhibiting free-radical polymerization;wherein the at least one group of the stabilizer compound capable ofsorption at the surface of the substrate is:

wherein R¹ is C₁-C₁₈ alkyl, C₇-C₁₁ aralkyl or C₆-C₁₀ aryl and Xrepresents a single bond, —O—, —NH— or —N(C₁-C₁₀ alkyl)-, (c) drying thelayer obtained in step (b); (d) applying to the dried layer obtained instep (c), a radiation-sensitive mixture comprising (i) at least onefree-radical polymerizable monomer, oligomer, polymer or mixture thereofwith at least one ethylenically unsaturated group, (ii) at least onephotoinitiator or sensitizer, which is capable of absorbing radiation ofa wavelength in the range of 250 to 1,200 nm, (iii) a solvent or solventmixture, and (iv) optionally at least one additive comprisingcoinitiators which form free radicals after the excitation of thephotoinitiator or sensitizer with radiation of a wavelength of 250 to1,200 nm, binders, dyes, plasticizers, chain transfer agents, leucodyes, inorganic fillers, surfactants or polymerization inhibitors notsuitable for sorption at the surface of the substrate; and (e) dryingthe layer obtained in step (d).
 11. The process according to claim 10,wherein the process further comprises the step of applying anoxygen-impermeable overcoat to the layer dried in step (e). 12.Lithographic printing form produced by a process comprising the stepsof: (a) providing a radiation-sensitive element comprising: (1) asubstrate with at least one hydrophilic surface and (2) aradiation-sensitive coating on at least one hydrophilic surface of thesubstrate, wherein the coating comprises: (i) at least one free-radicalpolymerizable monomer, oligomer, polymer or mixture thereof with atleast one ethylenically unsaturated group each, (ii) at least onephotoinitiator or sensitizer, which is capable of absorbing radiation ofa wavelength in the range of 250 to 1,200 nm, (iii) at least onestabilizer compound comprising at least one group capable of inhibitingfree-radical polymerization, and at least one other group capable ofsorption at the hydrophilic surface of the substrate, wherein the atleast one group of the stabilizer compound capable of sorption at thesurface of the substrate is:

wherein R¹ is C₁-C₁₈ alkyl, C₇-C₁₁ aralkyl or C₆-C₁₀ aryl and Xrepresents a single bond, —O—, —NH— or —N(C₁-C₁₀ alkyl)-, and (iv)optionally at least one additive comprising coinitiators which form freeradicals after the excitation of the photoinitiator or sensitizer withradiation of a wavelength of 250 to 1,200 nm, binders, dyes,plasticizers, chain transfer agents, leuco dyes, inorganic fillers,surfactants or polymerization inhibitors not suitable for sorption atthe surface of the substrate; (b) image-wise exposure of the elementwith radiation of a wavelength adjusted to the absorber contained in theradiation-sensitive coating; (c) optionally heating the irradiatedelement; (d) removing the unexposed areas of the coating with an aqueousalkaline developer; and (e) optionally heating the developed printingform or subjecting it to overall exposure or both.